I haven't taken the GRE yet (taking it thursday!) but I've found Barron's book to be very helpful. It really helped my score and on the last practice exam (from Princeton Review) I took I got a 163 on the verbal which I was very happy with. Almost all the words in it were in that book and the few I got wrong were more reading comprehension issues, not vocabulary issues.
Edit: Also I should mention that I was able to get it for free just by looking at the free things being offered on craigslist. I got that along with 2 other GRE study books from someone who had finished taking it. I would give that a try and see if you can't score any free study material.